This invention relates to a process and apparatus for producing a metalliferous concentrate from a particulate feed material. The term "metalliferous concentrates" is used herein to denote a concentrate which is substantially richer in precious metals (i.e. gold, silver and the six metals of the platinum double triad) than the particulate feed material from which it is derived. The process and the apparatus of the invention are especially intended for treating particulate feed material containing particles of various sizes, and are particularly intended for use in producing an auriferous concentrate from a particulate feed material containing low concentrations of gold, of the order of 0.1 oz. per short ton of feed material.
Gold is often found in so-called placer deposits, that is deposits which have been formed by stream action over long periods, perhaps millions of years. Such placer deposits comprise mixtures of sand, gravel and boulders, with the gold (and sometimes other precious metals) being present in the form of very fine particles mixed with the sand and gravel. Because the placer material has been deposited by stream action over a very long period, there is a tendency for the heavier particles, including the gold, to concentrate at certain levels and in discrete areas rather than to be distributed uniformly throughout the deposit. Placer deposits typically contain an average of about 10% of magnetite (Fe.sub.3 O.sub.4), a relatively dense (specific gravity about 5), dark colored granular material known as "black sand". Because both the magnetite and the gold are considerably denser than other constituents of the placer deposits, the magnetite or black sand tends to separate along with the gold particles both in situ in the placer deposit and during processing by simple gravity separation.
The extraction of gold from placer deposits presents great difficulties. The concentration of gold in the placer deposit is usually low, typically of the order of 0.03-0.2 oz/per ton, and the gold is distributed in a highly non-uniform manner because of the heterogenous nature of the placer deposits so that it is difficult and expensive to obtain samples and assays that are reasonably representative of the entire deposit. Furthermore, investigations have revealed that present day commercial processes for extracting gold from placer deposits frequently discard a significant proportion of the gold in the deposits, usually in the black tailings produced during processing. Because the gold concentration in the deposits is so low, it is necessary to produce from the deposits a metalliferous concentrate greatly enriched in gold and any other precious metals which are present; this metalliferous concentrate can then be subjected to further chemical processing familiar to those skilled in the art to extract the precious metals. Moreover, whatever process is adopted for producing the metalliferous concentrate must be able to handle large quantities of feed material cheaply; assuming a gold price around U.S. $400 per ounce, and allowing for the costs of further refining of the concentrate, we estimate that typical placer deposits can only be economically mined if the processing costs do not exceed about $5-6 per ton of placer deposit material.
This invention seeks to provide a process for producing a metalliferous concentrate from a particulate feed material which can be economically applied to a typical placer deposit.